Processing of exhaust gas is of increasing importance since highly populated areas accumulate impurities from exhaust gases that are harmful to human health and are unsightly. Of specific interest is processing of combustion exhaust, and more specifically, processing of automobile combustion exhaust as well as industrial process combustion exhaust.
Glow discharge or corona discharge has been exploited for exhaust gas processing. Japanese patent JP 6-269635, S. Nishida et al., shows equipment for NO.sub.x removal from combustion exhaust. Nishida et al. discuss a chamber containing many electrodes in the form of dielectric coated rods placed parallel to one another and perpendicular to the flow of combustion exhaust within the chamber. Nishida et al. improved the chamber by forming rectangular subchambers with walls of the subchambers made of dielectric coated sheet electrode material. The walls are parallel to the flow of combustion exhaust for reducing pressure drop past the electrodes and accommodating higher exhaust gas flow rates. Complete destruction of NO.sub.x is reported to require electrical power from about 1100 Watt to about 2100 Watt, for gas flow rates of about 1000 m.sup.3 N/h and NO.sub.x concentration from about 20 ppm to about 200 ppm (4-7.6 W/l/sec).
Another Japanese patent, JP 7-116460, S. Nishida et al., describes an exhaust gas reactor having a flat electrode and a sawtooth electrode juxtaposed with exhaust gas flowing therebetween. High voltage applied to the electrodes creates a charge in the exhaust gas. The reactor is capable of removing about 80 to 90% of the NO.sub.x at a flow rate of about 100 l/min for concentrations of NO.sub.x from about 50 ppm to about 200 ppm. However, when the O.sub.2 concentration in the exhaust gas exceeds several per cent, the NO.sub.x is oxidized and the NO.sub.x removal rate is degraded. By adding a carbon layer to the downstream portion of the flat electrode, increased concentrations of O.sub.2 do not affect NO.sub.x removal. However, the carbon layer becomes oxidized and eventually requires replacement.
A third Japanese patent, JP 5-115746, S. Nishida et al., discusses a concentric electrode chamber with an outer cylinder electrode and an inner rod electrode with exhaust gas flowing through the annulus therebetween. It is stated that this concentric electrode chamber is limited to a flow rate of about 60 l/min before the glow discharge is extinguished. Nishida et al. introduce a sawtooth with pitch of the saw tooth increasing in the direction of flow to maintain the glow discharge at flow rates as high as 1000 l/min. The amount of electrical power consumed to achieve 100% NO.sub.x destruction for that flow rate was not reported.
A U.S. Pat. No. 4,954,320 to Birmingham et al., discusses a concentric electrode air purification device wherein the annulus between the electrodes is filled with dielectric beads having a dielectric constant greater than about 33. Electric power consumed is reported as about 10 kW and near 100% destruction of cyanogen and phosgene at flow rates of 2.6 standard cubic feet per minute (scfm) (1.23 l/s) and 5.5 scfm (2.6 l/s) respectively (3.8-9.3 kW/l/s).
A second U.S. Pat. No. 5,254,231 to Heath et al., discusses a concentric electrode gas modification system wherein the annulus between the electrodes is filled with dielectric beads having a dielectric constant of less than 33. Electric power consumed is estimated to be from 60 to 700 Watt for nearly 100% destruction of 205 ppm trichloroethylene in air at 1.8 scfm (51 l/s) (1.2-13.7 W/l/s). Although not specifically reported, the pressure drop across the dielectric beads is significantly greater than the pressure drop through an open annulus.
Accordingly, there remains a need for an exhaust gas processing method and apparatus that achieves near 100% impurity removal with both reduced electrical power and reduced pressure drop of exhaust gas flow through an exhaust gas processing chamber.